[sdiy] linear vactrol driver circuits

[Tom Wiltshire]

Harry is right. The vactrol's LDR response time is being used as a  
basic filter to turn the PWM signal back into an analogue level. As  
such, you need a high enough PWM frequency that you won't hear it.  
However, that isn't hard to organise.

A chap on diystompboxes.com recently built a tremelo using my VCLFO  
and this technique. There are audio samples available if you'd like  
to hear the results. There's no noise at all, but the PWM frequency  
is around 19KHz.

	http://www.diystompboxes.com/smfforum/index.php?topic=77471

Schematic at:

	http://img196.imageshack.us/i/schematicpiculustrem10.jpg/

In short, Harry's right to be cautious, but it can easily be done  
successfully.

T.



On 27 Jul 2009, at 17:35, HARRY BISSELL wrote:

> PWM must be high enough in frequency not to introduce any noise...
> vactrols are slow but even if a high frequency is MANY dB down, you
> may hear it...
>
> H^) harry
>
> ----- Original Message -----
> From: Tom Wiltshire <tom at electricdruid.net>
> To: Julian <julian at port23.co.uk>
> Cc: synthdiy DIY <synth-diy at dropmix.xs4all.nl>
> Sent: Mon, 27 Jul 2009 12:06:54 -0400 (EDT)
> Subject: Re: [sdiy] linear vactrol driver circuits
>
>
> On 27 Jul 2009, at 15:56, Julian wrote:
>
>>
>> Ive been considering using vactrols to add CV control, but am
>> wondering what my best approach is for driving them.
>>
>> Im thinking along the lines of a CV input jack, and a front panel
>> pot, and an opamp mixer / driver for the led, but am wondering as
>> to the best way to get away from the log response of the vactrol.
>>
>> Im sure this must be a common issue when using vactrols, so im
>> wondering what other people do?
>
> I don't know what other people do, but I've been experimenting with
> driving the LED in a vactrol with a PWM signal. This is supposed to
> give a more linear relationship between the driving signal and the
> LED brightness. I was using a microcontroller, but you could easily
> use a ramp or triangle wave oscillator and a comparator for an analog
> PWM solution.
>
> T.
>
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>
>